1. Field of the Invention
The present invention relates in general to computed tomography as used in medicine to examine patients, particularly to a method for image reconstruction for sectional dose-reduced spiral scanning.
2. Description of the Prior Art
Using modern medical diagnostic techniques such as X-ray computed tomography, it is possible to acquire image data for an object to be examined. The examined object is generally a patient.
X-ray computed tomography (CT for short) is an X-ray imaging technique that differs fundamentally in terms of the image formation from classic X-ray tomographic imaging techniques. With CT images, transverse image slices are obtained, i.e., images of body slices that are oriented essentially perpendicularly to the body axis. The tissue-specific physical quantity represented in the image is the distribution of the attenuation values of X-ray radiation μ(x, y) in the slice plane. The CT image is obtained by reconstruction from the two-dimensional distribution of μ(x, y) from numerous different angles of view (projections).
The projection data are determined from the intensity I of an X-ray beam after passing through the slice to be imaged and from its original intensity IO at the X-ray source according to the absorption law as follows:
      ln    ⁢                  I        0            I        =            ∫      L        ⁢                  μ        ⁡                  (                      x            ,            y                    )                    ⁢              ⅆ        l            
The integration path L represents the path of the observed X-ray beam through the two-dimensional attenuation distribution μ(x, y). An image projection is then composed of the measured values that were acquired with the X-ray beams for a viewing direction of the line integrals through the object slice.
The projections emanating from a wide variety of different directions (characterized by the projection angle Φ) are obtained using a combined X-ray tube detector system (gantry) that rotates in the slice plane about the object. The most commonly used devices currently are the type is known as “fan-beam devices” in which a tube and an array of detectors (a linear arrangement of detectors with a defined width S) rotate in the slice plane jointly about a rotational center which is also the middle of the circular measurement field. “Parallel radiation devices” are also known, but exhibit very long measurement times are not explained in detail herein. It must be noted, however, that a transformation of fans—to parallel projections and vice versa—is possible so that the present invention that is to be explained based on a fan-beam device is equally applicable for parallel-beam devices.
FIG. 6 schematically shows a computed tomography device for a fan-beam technique. In this device, an X-ray tube 7 and a radiation receiver 13 (an array of linearly arranged detector elements) rotate—the two together being known as a “gantry”—jointly around a rotational center which is also the center of the circular measurement field 5 (gantry opening) and in which the patient to be examined 1 is located on a patient bed 2. In order to be able to examine different parallel planes of patient 1, the patient bed can be displaced along the body's longitudinal axis. As can be seen from FIG. 6, in CT imaging transversal image slices will result, i.e. images of body slices oriented essentially perpendicularly to the bodily axis. CT requires projections at many angles φ. To generate a slice image, the beam cone emitted by the X-ray tube 7 is gated such that a planar radiation fan arises which traces one-dimensional central projections of the irradiated slice. For exact reconstruction of the distribution of the attenuation values μz(x, y) (where z is the position on the body's longitudinal axis), this radiation fan must be perpendicular on the rotation axis and also must be spread wide enough to completely cover, from each projection direction φ, the slice of the measurement object in the beam's field of view. The radiation fan penetrating the object is detected by detectors that are linearly arranged on a circle segment. With conventional devices, there are up to 1000 detectors. The individual detector responds to the incident beams with electrical signals the amplitude of which is proportional to the intensity of these beams. With detectors known as “multi-row detectors”, a number of detector rows are arranged in parallel.
Each individual detector signal belonging to a projection φ is picked up in each case by an electronic measurement circuit 15 and forwarded to a computing unit (computer or system computer) 16. With the computing unit 16, the measured data can now be processed in a suitable manner and displayed in the form of an X-ray image in units known as “Hounsfield units” on a monitor 14.
Larger volumes of the examination subject generally are picked up using spiral scanning (spiral scan). With spiral scanning, the gantry rotates with the radiation source continuously around the examination subject while the patient bed is displaced relative to the gantry continuously along a system axis (generally the patient's longitudinal axis, or z axis).
The radiation source thus delineates, referenced to the examination subject, a spiral path until the volume determined prior to the examination has been scanned. Based on the spiral data acquired in this manner, images for the individual slices then can be computed.
The parameter selection in spiral CT corresponds largely to that used in conventional CT.
As an additional parameter in spiral scans, the table feed d in mm per 360° rotation must be selected. The ratio of the table feed d to the slice collimation M·S (the product of the number M of detector rows and the width S of the detector row) as a dimensionless quantity is generally referred to as the pitch or pitch factor p:
  p  =      d          M      ·      S      
Generally, pitch values between 1 and 2 are chosen. The larger the pitch, the faster the scan volume is covered.
As a general rule, the patient dose depends both in conventional CT and in spiral CT on many parameters, besides the technical properties of the CT system and the selected examination parameters, particularly also on the patient size and the selected anatomical examination region.
Because CT imaging is based on the attenuation or absorption of X-ray radiation in organic tissue, during the irradiation an energy transfer to tissue results (radiation dose), which can lead to cell damage.
A goal in CT imaging is to keep the dose during the CT imaging as low as possible for the patient. Particularly, it is important to ensure that particularly radiation-sensitive organs receive as little exposure as possible. According to “ICRP: Publication 60—Recommendation of the International Commission on Radiological Projection; Pergamon Press, Oxford, 1990”, particularly radiation-sensitive organs include, i.e., the gonads, female mammary gland, thyroid gland and the eye lens.
Conventionally, the dose for the patient in CT imaging usually is reduced, for example, by reducing the tube current. A simple reduction in the tube current reduces the dose for the patient, but the image quality is degraded to the same extent.
The influence of a dose reduction on the image quality cannot be ignored. A dose reduction technique that has been further developed in this regard involves an attenuation-dependent tube current modulation (CAREDose, Gies, Kalender, Wolf, Suess: Dose reduction in CT by anatomically adopted tube current modulation, 1 Simulation Studies Med. Phys. 26 (11): 2231-2247, 1999). In this technology, for projections with a high attenuation—e.g., laterally along the shoulder axis of the patient—the tube current is slightly boosted; for projections with a low attenuation—e.g., from anterior to posterior (a.p.) or vice versa (p.a.)—the tube current is greatly reduced. Use is made of the fact that the image point noise is determined primarily by the projections in which the attenuation through the object is high. A reduction of the tube current in the projections with low attenuation thus has no negative influence on the image quality.
CT fluoroscopy proceeds in a similar manner, wherein data are continuously acquired and immediately reconstructed from the same slice. The imaging takes place without any movement of the table. In this manner, it is possible to track the position of a medical instrument in the patient, for example, in the context of a centesis or biopsy. A current image is always available to the physician performing the examination. To protect the hand of the physician from excessive radiation exposure, in a special embodiment of fluoroscopy (HandCARE), the X-ray radiation is greatly reduced, or switched off totally, in the anterior-posterior direction. This method (HandCARE) thus aims to minimize the radiation dose to the physician, i.e., the dose to the hand of the physician. The missing data for projections with reduced or missing radiation are reconstructed in HandCARE using suitable algorithms.
As already mentioned, the data acquisition takes place in the described techniques in a slice-by-slice manner without selective dose reduction.